1. Field of the Invention
The present invention relates to a fuel cell stack formed by stacking a plurality of unit fuel cells.
Priority is claimed on Japanese Patent Application No. 2005-331232, filed Nov. 16, 2005, the content of which is incorporated herein by reference.
2. Description of the Related Art
In a known fuel cell, an anode and a cathode are provided on either side of a solid polymer electrolyte membrane so as to form a membrane electrode assembly. The membrane electrode assembly is placed between a pair of separators, thereby forming a unit fuel cell (called a “unit cell” below) having a plate shape. Generally, a specific number of unit cells are stacked so as to form a fuel cell stack.
In this fuel cell, hydrogen ions generated at the anode by catalytic reaction move through the solid polymer electrolyte membrane to the cathode, and an electrochemical reaction occurs between the hydrogen ions and oxygen at the cathode, thereby generating electric power. In this process, water is also generated (which is called “generated water” below). This reaction proceeds along the flow of reaction gases, from the upstream side to the downstream side. In addition, electric power generation also generates heat; thus, the fuel cell is generally cooled using a coolant flowing through the fuel cell, so as to continue the power generation.
In an example of a coolant passage, each separator having protruding lines, each line winding like waves, is formed by press-molding a metal plate, and is arranged in close contact with the anode or cathode of each membrane electrode assembly, so as to use the space between the protruding lines and the anode or cathode as a reaction gas passage (i.e., a fuel gas passage or an oxidant gas passage) and to use the space formed between adjacent separators as a coolant passage. In this case, the flow direction of each reaction gas is perpendicular to that of the coolant passage (see Japanese Unexamined Patent Application, First Publication No. 2003-338300).
As shown in FIG. 8, when the reaction gas is made to flow from the upper side to the lower side of a fuel cell stack S, that is, downward in the vertical direction (or the gravitational direction), the above-described reaction proceeds from the upper side toward the lower side, and water generated during the reaction also flows from the upper side toward the lower side. Therefore, the relative humidity in each reaction gas passage is higher at the lower side in comparison with the upper side.
When such a difference in humidity occurs in the fuel cell stack, the electric power generation cannot be performed uniformly.